This invention relates in general to power amplifiers and, more particularly, to a very low distortion power amplifier or servo-amplifier which includes an operational amplifier but which is not limited by the inherent limitations of the operational amplifier.
One preferred mode of operation of the power amplifier of the present invention is in a stereo system and, hence, the problems of the prior art and the benefits of the present invention will be described in that context. It is to be understood, however, that there are many other usages for the present invention as will be explained in greater detail.
In a stereo system, the function of a power amplifier is twofold. First, the power amplifier serves as an impedence match between the output from the pre-amplifier, which is typically 10 k ohms and the speaker which is typically 8 ohms. Secondly, the power amplifier takes the output signal from the pre-amplifier, which pre-amplifier provides both frequency response and some limited voltage amplification, and the power amplifier amplifies the voltage by approximately 15 times and the current by approximately 1,000 times to provide power amplification of 15,000 times the output of the pre-amplifier.
General purpose operational amplifiers are very well known. It is also known that they have certain inherent limitations. For example, they have a limited frequency response and a very limited slew rate. Yet another problem is that the output voltage swing cannot exceed the supply voltage. Since the supply voltage is typically in the range of 15 volts, this results in a very limited power output rating for the operational amplifier.
Based upon all these inherent limitations of a general purpose operational amplifier, they cannot be used in a power amplifier except for a most limited output power rating and with frequency distortion of the output signal.
Thus, the present invention provides an improved power amplifier utilizing a general purpose operational amplifier without the aforementioned limitations.